A few things. I (purposefully) chose to gain about 10 pounds which I really needed, I was gaunt. Quitting smoking can cause differences to thyroid, but it's usually in the direction of needing more thyroid, not less (although, not always). I needed to up my thyroid meds when I started vaping and that made a huge difference. It was reflected in my symptoms but also bloodwork. I am considering increasing it more, but really not by much, I think I am almost where I need to be.
I was lethargic and tired at first (W/D from smoking plus I had a lot of oral surgery) but that passed and now my energy levels are through the roof! I really had to eat a LOT and work hard to gain that weight, though, fortunately food tastes fantastic. I'm stabilized where I want to be but I eat a fairly healthy diet and exercise regularly.
I think in many cases, weight stays the same or only increases slightly, depending on a variety of other factors as well, though. Also, I highly doubt that what I am inhaling and exhaling is giving me calories though it might if I ate it.
I'd say get checked out by a doc, get your thyroid tested and rule out the usual suspects. If everything's fine health wise, it will be a matter of discipline to exercise regularly and eat a healthy diet. I don't eat sweets as much any more, but I eat a LOT, and vaping some dessert flavors has helped with that, I think.
Good luck,
Anna